双宾语中的动词是不是原型 是hear her sing 还是hear her sings

hear her sing 听到她唱过
hear her singing听到她正在唱

都可以，看说话者要表达的意思，比如：
I often hear her sing in the afternoon.我经常在下午听到她唱歌
I heard her singing at 3:00pm yesterday.我昨天下午三点听到她正在唱歌

hear her sing,sing 这里是不带to的动词不定式,只能用原型,也可以是hear her singing,强调正在唱歌
sings是错的

两种用法：hear sb doing sth；hear sb do sth。前者是听到某人正在做某事；后者是听到某人做过某事。